ABSTRACT ? Project 3 The progression of primary brain tumors in children from low grade (LGGs) to lethal high grade gliomas (HGGs) typically occurs over a 10 year period and is not prevented by traditional therapies. Although immunotherapy seems to be an ideal alternative therapeutic approach for LGG tumors due to the ample time window between resection and recurrence during which anti-tumor immunity can be stimulated, attempts to develop the approach have been unsuccessful. The most common mutation found in LGGs, IDHR132H, is located in the isocitrate dehydrogenase 1 (IDH1) gene and is ubiquitously expressed in all tumor cells. Unfortunately, although its prevalence and ubiquity make IDHR132H an excellent tumor vaccine target, it has proven to be very poorly immunogenic, especially in generating CTL responses. This Project aims to overcome this hurdle by establishing proof of principle for a novel monocyte vaccination strategy for IDHR132H+ gliomas. The approach exploits an endogenous antigen (Ag) cross-presentation presentation pathway that uses resident splenic dendritic cells (DC) to significantly improve the survival of mice bearing intracranial IDH1R132H+ CT2A tumors. Our preliminary data suggest that this strategy may stimulate anti-IDH1R132H CD8+ T cell responses. Building on these findings and additional preliminary data showing that Flt3L markedly improves monocyte vaccine efficacy in mouse models of melanoma, the Project will test the hypotheses that monocyte vaccination can induce effective anti-IDH1R132H CTL responses in mice, that these responses can be increased by Flt3L administration, and that monocyte vaccination will safely stimulate robust anti-IDH1R132H immune responses in humans. The objectives of this project are to determine the specific mechanisms by which monocyte vaccination inhibits IDH1R132H glioma growth (Aim 1), to determine if Flt3L improves the efficacy of monocyte vaccination for IDH1R132H-expressing gliomas in mice (Aim 2), and to determine if IDH1R132H monocyte vaccination is safe and immunogenic in humans (Aim 3). These studies are expected to provide proof of principle that an entirely novel vaccine strategy, monocyte vaccination, possibly in combination with Flt3L administration, provides superior efficacy to alternative strategies in animal models of IDH1R132H gliomas, determine the mechanisms by which this efficacy is achieved, and demonstrate that this strategy is safe and immunogenic in a relevant patient population. If successful, this Project will provide the rationale and critical information needed for future studies of monocyte vaccination, which has the potential to significantly improve outcomes in patients with IDH1R132H-expressing gliomas.